This invention relates generally to photovoltaic devices and more particularly to a building material which is capable of generating electrical power. Most specifically, the invention relates to a power generating material which is flexible, lightweight and self-adhesive.
Photovoltaic energy is becoming a very significant source of electrical power. This is because problems of scarcity and safety have limited the use of fossil and nuclear fuels, and recent advances in photovoltaic technology have made possible the large scale manufacture of low cost, lightweight, thin film photovoltaic devices. It is now possible to manufacture large scale, thin film silicon and/or germanium alloy materials which manifest electrical and optical properties equivalent, and in many instances superior to, their single crystal counterparts. These alloys can be economically deposited at high speed over relatively large areas and in a variety of device configurations, and as such they readily lend themselves to the manufacture of low cost, large area photovoltaic devices. U.S. Pat. Nos. 4,226,898 and 4,217,364 both disclose particular thin film alloys having utility in the manufacture of photovoltaic devices of the type which may be employed in the present invention. However, it is to be understood that the present invention is not limited to any particular class of photovoltaic materials and may be practiced with a variety of semiconductor materials including crystalline, polycrystalline, microcrystalline, and noncrystalline materials.
The power generated by a photovoltaic device is proportional to the illumination incident thereupon, and if relatively large amounts of power are to be generated, fairly large collection areas are required. The roof and upper story areas of building structures are well illuminated and are generally not put to productive use. For some time now it has been known to place photothermal and photovoltaic collectors on the top portions of buildings. Such prior art approaches include mounting of photovoltaic panels on buildings through the use of rigid mounting brackets. In other instances, photovoltaic devices are configured into roofing tiles. The use of such materials requires special installation techniques and hardware. In addition, such prior art devices and mounting hardware tend to be heavy; therefore, building structures must be reinforced to accommodate their use. In fact, building codes in many areas mandate reinforcement and redesign of roofing structures if weight loadings in excess of two pounds per square foot are added thereto.
In addition to being heavy, prior art devices tend to be rigid, and this rigidity, together with weight, complicates shipping, handling and installation. Further problems of installation occur when photovoltaic devices are mounted, since mounting typically requires use of special frames and/or fasteners such as nails and screws which penetrate the photovoltaic device or mounting structure. The use of penetrating fasteners is complicated by the fact that such fasteners should not penetrate the photovoltaically active portions of a roofing material. Therefore, complex mounting hardware is frequently required to assure that photovoltaic devices are retained on a building structure with sufficient integrity to resist storm conditions. The prior art has implemented a number of approaches to the fabrication of photovoltaic roofing materials. For example, U.S. Pat. Nos. 5,092,939; 5,232,518 and 4,189,881 disclose photovoltaic roofing structures of the batten and seam type. U.S. Pat. No. 4,860,509 discloses roll type roofing material having photovoltaic devices incorporated therein. U.S. Pat. Nos. 5,575,861 and 5,437,735 disclose photovoltaic shingles.
Despite the advances made in photovoltaic roofing materials, there is still a need for a photovoltaic building material which is light in weight, flexible enough to be installed over irregular surfaces, and capable of being retained onto a building structure without the use of significant numbers of fasteners or extraneous hardware. In addition, such material should be resistant to the spread of fire.
Photovoltaic building materials frequently include polymeric encapsulant, asphalt based sealers, fibrous support materials and the like, all of which can be flammable, particularly given the fact that such materials are usually used in the form of relatively thin sheets. Building codes generally set flammability standards for construction materials. Such standards specify the rate at which flame will spread across a material. When relatively flammable materials are incorporated into building structures, insurance rates for the buildings may be increased to the point that construction is prohibitive. Also, in many cases, building codes are strict enough to preclude the use of flammable materials. Therefore, it is further desirable that any photovoltaic building material have a relatively low rate of flame spread.
The present invention provides a low cost, flexible photovoltaic building material which is lightweight, self-adhesive, and which can be easily and fixedly attached to a variety of building structures. The material of the present invention is relatively low in cost and can be shipped in a rolled form. In addition, the material has a low flammability and can also be configured to include firebreak structures which limit flame spread. Additionally, the self-adhesive nature of the material of the present invention allows the material to be adapted to a number of other applications including lightweight power generating modules, vehicles, consumer products, and the like. These and other advantages of the present invention will be apparent from the drawings, discussion and description which follow.
There is disclosed herein a photovoltaic building material which comprises a generally planar, flexible substrate having a first and a second opposed side. A photovoltaic device, operative to generate a current in response to the absorption of incident light, is supported on the substrate so that the bottom side of the photovoltaic device faces the first side of the substrate and the top, light incident, side of the photovoltaic device faces away from the substrate. An encapsulant material covers the light incident side of the photovoltaic device and affixes that device to the substrate. At least that portion of the encapsulant material which covers the light incident side of the photovoltaic device is light transmissive. The photovoltaic building material further includes a body of contact adhesive covering at least a portion of the second side of the substrate. The contact adhesive is employed to affix the module onto a building structure. The building material may be configured to have a weight of no more than two pounds per square foot.
In specific embodiments, the contact adhesive is covered by a sheet of release material. The adhesive, in one preferred embodiment, comprises a rubberized asphalt. The substrate may be any typical roofing material such as metallic foils, polymeric membranes, polymer impregnated fiber mats, and the like taken either singly or in combination.
In some preferred embodiments, the module includes a plurality of separate photovoltaic devices disposed on the substrate in a spaced apart relationship. Firebreak members may be disposed so as to span the module, preferably between discrete photovoltaic devices.
In some specific applications, the material of the present invention may be adhered to a lightweight support member, such as a sheet of polymeric material, so as to produce a lightweight module. This module may further include ancillary equipment such as inverters, power controllers, charge controllers, terminal assemblies, power storage devices and the like affixed thereto or integrated therewith.